Aromatic dicarboxylic acids are commonly manufactured by the catalytic oxidation of a hydrocarbon precursor in an organic solvent. An example is terephthalic acid (TA), which is widely used in the manufacture of polyesters, such as poly(ethylene terephthalate) (PET). The TA required as a reactant for PET production is known as “purified terephthalic acid” (PTA) and generally contains over 99.97 wt %, preferably over 99.99 wt %, of terephthalic acid, and less than 25 ppm 4-carboxybenzaldehyde (4-CBA). On the commercial scale, PTA suitable for use in PET production is generally prepared in a two-stage process. First, p-xylene is oxidized (e.g. in air) in the presence of a metal catalyst (e.g. a cobalt and/or manganese salt or compound) to provide “crude terephthalic acid” (CTA), as described in, for example, U.S. Pat. No. 2,833,816. Second, the CTA produced by this oxidation reaction is then purified, as it is typically contaminated by impurities such as 4-CBA, p-toluic acid, and various coloured impurities that impart a yellowish colour to the TA. Purification of the CTA typically requires at least one chemical transformation (e.g. hydrogenation) in addition to at least one physical procedure (e.g. crystallization, washing, etc.) to yield PTA.
PTA is generally considered to be a commodity item, with several million tonnes being produced annually, and it is therefore desirable for manufacturers to reduce their costs to maximise the economy and efficiency of PTA production. This can be achieved both by reducing capital costs (e.g. equipment costs) and variable costs (e.g. costs associated with waste disposal, use of starting materials, organic solvent, heating fuel and demineralised water).
EP-A-0962442 discloses a process in which an aqueous purification mother liquor from which PTA crystals have been separated is guided to a cooling vessel and cooled to precipitate organic compounds, such as p-toluic acid. These organic compounds are separated in a separator from the mother liquor, which is then subjected to a p-xylene extraction and used as slurrying water and as reflux to a middle portion of a distillation column for separating acetic acid and water. However, this cooling and filtering process not only results in the loss of heat from the aqueous purification mother liquor and thus from the distillation column, it also results in the removal of valuable organic compounds, including compounds that may be oxidised to TA as well as minor amounts of TA itself, from the reaction system.
It is an object of the present invention to provide a more economic and efficient process and apparatus for the manufacture of aromatic dicarboxylic acids and, in particular, to provide a process and apparatus that overcome the aforementioned disadvantages. Further objects will be apparent from the description below.